NPLC-LED Report Summary

Final Report Summary - NPLC-LED (IMMERSION COOLING OF SUSPENDED AND COATED NANO-PHOSPHOR PARTICLES FOR ENHANCED THERMAL AND OPTICAL EXTRACTION OF LIGHT EMITTING DIODES FOR GENERAL ILLUMINATION)

LED systems are limited with their efficacy that is primarily due to the optical losses. There are a number of optical losses in a typical LED system. Although they have been mentioned in a number of publications partially, there is no published research that presents the total optical loss scheme in SSL systems. Therefore there is a need for a research that can grasp all of the optical losses in the LED systems. However, the resultant inefficiency in the phosphor manifests itself in heat and can further reduce the phosphor efficiency. All of these factors in an LED package can be quite challenging and they must be coupled with all the factors including thermal design and reliability for the whole system to merge an optimal design. Even though optical design reaches its optimum, it cannot avoid local hot spots and temperature non-uniformities that needs be addressed.The inclusion of phosphor into a high brightness (HB) LED package is a complex task. In a typical 450 or 470 nm blue LED, YAG phosphor is introduced and combined with the blue emission to create what appears to the eye is white light. There are several factors that must be considered when adding the phosphor such as particle size, concentration, geometry and carrier medium. The common practice for phosphor carrier medium is typically a high index of refraction silicone but the geometry of the phosphor is a primary design variable and can be classified as dispersed, remote, and local (see Figure 1). In each case, the geometry greatly affects the ultimate optical output of the LED color qualities and conversion efficiency.Since there is limited information about individual losses in an LED system and there is no available correlation to predict the total losses, the proposed research is for filling the gap for this fundamental problem. In addition, after identifying the losses, LED effective liquid cooling is tackling a major challenge “hot phosphor losses” that provides unique information for both fundamental nano-fluid (phosphor based) heat transfer and improved efficacy.The introduction of liquid cooling with optically-transparent liquids has been the main focus of this project to reduce average chip temperatures and to improve the uniformity of chip and phosphor temperature, leading to higher light extraction efficiencies. Furthermore, this project involved a number of key research areas such as; liquid cooling with nano-phosphor fluids, optical losses due to phosphor white light conversion, computational modeling of heat transfer and optical behavior, and developing an experimental study to validate modeling results. Finally, measuring junction temperature to evaluate the performance of an LED is a good predictor of LED life and light quality. In his research, a method has been developed to measure the LED junction temperature and related junction temperatures. In addition, a further study with under the Raman Spectroscopy and Infrared (IR) imaging system has also been performed. In summary, this research involved both developing numerical models and validating with experimental research. Lumen extraction has been shown with this novel cooling method and a method to measure the LED chip temperature has also been developed and proven with the experimental finding. This study produced a PhD dissertation and 2 Master of Science thesis generating 11 international publications and two patents.